Solvation-Tuned Photoacid as a Stable Light-Driven pH Switch for CO2 Capture and Release

Photoacids are organic molecules that release protons under illumination, providing spatiotemporal control of the pH. Such light-driven pH switches offer the ability to cyclically alter the pH of the medium and are highly attractive for a wide variety of applications, including CO2 capture. Although photoacids such as protonated merocyanine can enable fully reversible pH cycling in water, they have a limited chemical stability against hydrolysis (<24 h). Moreover, these photoacids have low solubility, which limits the pH-switching ability in a buffered solution such as dissolved CO2. In this work, we introduce a simple pathway to dramatically increase stability and solubility of photoacids by tuning their solvation environment in binary solvent mixtures. We show that a preferential solvation of merocyanine by aprotic solvent molecules results in a 60% increase in pH modulation magnitude when compared to the behavior in pure water and can withstand stable cycling for >350 h. Our results suggest that a very high stability of merocyanine photoacids can be achieved in the right solvent mixtures, offering a way to bypass complex structural modifications of photoacid molecules and serving as the key milestone toward their application in a photodriven CO2 capture process.


pH-jump and reversibility i. Calculation of reversibility time
The reversibility time was estimated using extrapolation of the linear thermal relaxation region (first 10 minutes in the dark) to pH = pH GS (Figure S2, Table S1, Equations S1-S2).

Equation S2
ii. Custom photo-switching setup A custom-made optical cell was designed for photo-switching experiments (Figure S3).The cell comprises of a cuvette holder (black polyoxymethylene, POM), that contains a 2 x 2 x 2 cm quartz glass cuvette, 3 removable shutters (black slides) and optional gas in-and outlet (red fittings).The cell is closed by a lid (beige polyether ether keton, PEEK) that has an inlet for the pH electrode (black shaft), which is held and sealed by compression of an o-ring (top lid).The photo-switching setup is contained in a black box that is covered with tin foil to avoid any light exposure other than LED light.

pKa determination
S3,S4] Therefore, using UV-Vis titration we studied how the pKa GS of a mPAH changes in water-DMSO solvent mixture compared to pure water.Our experiments showed that the pKa GS of MCH decreases slightly from 6. 2 in pure water to 6.0, as calculated in a mixture with xDMSO= 0.15 (Figure S4).As the change in pKa is relatively small, we assume that the ratio MCH/MC -does not change considerably across the solutions studied, when starting with the same pH.However, further studies in other water-DMSO ratios are needed to fully understand the relationship between solvent structure and the related acidity of photoacids.

i. Procedure
For the determination of the pKa, the acidic and basic solutions of photoacid were prepared in a similar manner to pH-jump studies, arriving at the following compositions: • Basic solutions: 0.08 mM mPAH + 10 mM NaOH + 20 mM NaCl; • Acidic solutions: 0.08 mM mPAH + 10 mM HCl + 20 mM NaCl.Next, we performed a pH titration by adding the basic solution to the acidic one to modify the pH from 3 -9 with increments of 0.5.The UV-Vis absorption spectrum of a sample at each pH increment was measured.All samples were prepared and measured within 5 hours, for which a limited extent of degradation can be assumed S5 .
ii. UV-Vis absorption results and fitting The pKa was determined by analyzing the ratio between the absorption maximum of the protonated (   ) and deprotonated peaks (   − and    ) as a function of pH (Equation S3-S4).The pKa was assigned the pH value at which the fraction PAH/PA -is 0.5 (Figure S4C).ii.Thin layer configuration The measured pH-jump efficiency and reversibility time can be affected by lamp intensity, stirring efficiency, type of pH electrode, and light path length.Our measurements show that higher pH-jumps are observed in a cuvette with a path length of 5 mm compared to a cuvette with a 20 mm path length (Figure S7).Therefore, a thin layer configuration (5 mm), was used to determine the highest possible pH-jump.However, the pH electrode compatible with the small cuvette is not optimal for the long-term stability measurements, since the ceramic junction allows for the exchange of the filling solution with the photoacid solution (LE422, Mettler Toledo).Here, we again note that contributions and values of LJP for pH meter probes with different pH junctions types are not accounted for in current study, but can affect the readings of the pH meters.Therefore, measured pH values should be taken as approximate.For longterm stability measurements, the pH electrode with a gel electrolyte with minimal leakage was used (polyester junction, LE438, Mettler Toledo).However, due to the larger diameter of this pH electrode (12 mm), it was not compatible with the 5 mm path length cuvette.Consequently, long-term stability experiments were conducted in the larger cuvette, which do not allow for the highest achievable pH-jump.

Solubility i. Procedure
To study the solubility of merocyanine in water and DMSO, two types of solution series were evaluated: -Pre-mix series: first, a solvent mixture with the desired water-DMSO ratio was prepared.Then, in a septum-sealed, amber vial, pre-mixed water-DMSO solvent was added incrementally to a known amount of photoacid using a glass syringe (10 µl, Hamilton).After each addition, the mixtures were sonicated for 20 minutes to ensure complete dissolution of the photoacid.This process was repeated until all photoacid was dissolved.The vials were left at room temperature (~22°C) for a minimum of 3 days to ensure that the photoacid remained dissolved.
-Post-mix series: here, the solubility was evaluated following two different protocols (described below).
Incremental: first, a saturated solution of photoacid in pure DMSO was prepared (16.4 mM).Then, this solution was added to water in appropriate amounts to achieve the target water-DMSO ratio.If precipitation was observed upon addition, the process was repeated with a lower starting concentration in DMSO.
UV-Vis: first, for each water-DMSO ratio, UV-Vis spectra of solutions with known concentrations between [0.01;0.06]mM were collected.Then, a calibration curve was obtained by plotting the absorption maximum as a function of the known photoacid concentration and its linear fitting (y=ax) (Figure S8).Second, a saturated solution of photoacid in pure DMSO was prepared by adding excess amount of photoacid to DMSO.Then, the supernatant was passed through a syringe filter (PTFE, 0.45 µm, 13 mm, Fisherbrand) and added to water to achieve the target water-DMSO ratio, if precipitation occurred the syringe filtration process was repeated.Third, the saturated solutions were diluted by a known amount of the corresponding solvent mixture, to reach the range between [0.01;0.06]mM (in the linear dynamic range of the UV-Vis absorption).Finally, from the absorption maxima of the diluted sample and the calibration curve, the exact concentration of the diluted sample was obtained.This value was then used together with the known dilution factor, to back-calculate the concentration of the saturated solution for each water-DMSO ratio.

ii. UV-Vis absorption calibration curves and solubility result
Calibration curves were obtained by linear fitting (y=ax) of the absorption maxima of the MCH peak (right panels in Figure S8).Fitting parameters are stated in table S2.  iii.Higher pH-jump with higher solubility  6. Photochemical CO2 release detection i. Procedure CO2 release experiments were carried out with 20 mL solutions of 0.2 mM MCH in water and 5.3 mM in xDMSO = 0.15 with molar equivalents of KHCO3.The solution was placed in a custom-made optical cell that contained gas in-and outlet valves, pH-meter, magnetic stirbar, window for LED light (5 cm diameter) and a CO2 sensor (SCD41, Sensirion) in the headspace.The remaining headspace volume was 0.9 mL.At the beginning of each experiment, the headspace was flushed with 100 ml/min N2 for 3 minutes.Next, the in-/outlet valves were closed and the pH and CO2 sensors were left to stabilize for at least 30 minutes.To evaluate the photochemical release of CO2 the solutions were subject to three cycles of: 30 minutes of light and 30 minutes of darkness in the case of water solvent, and 20 minutes of light and 60 minutes of darkness in the case of the water-DMSO mixture.After each cycle of light, the valves were opened and the headspace was purged with N2 to avoid any re-absorption of prior released CO2.
ii. Calibration curve To quantify the CO2 release, the CO2 sensor was calibrated by purging different concentrations of CO2/N2 mixtures, controlled by mass flow controllers into the headspace of the optical cell (Figure S12A).The calibration curve was obtained by plotting the measured CO2 concentration by the sensor (ppmsensor) against the MFC input concentration (ppmMFC) and applying a linear fit (Figure S12B, Equation S5).
= −0.11164+ 5.855 *   Equation S5 iii.Data During experiments, CO2 release was continuously detected by the CO2 sensor in the headspace.The measured sensor data was first converted to calibrated data by solving equation S5 for ppmMFC to obtain actual gas concentrations.Then, the resulting gas concentration was converted to molar values by multiplying the headspace volume (Equation S6) and finally solving the ideal gas law (Equation S7).

Equation S7
Where VCO2 is the actual measured CO2 volume in m 3 , Vheadspace (in m 3 ) corresponds to the 0.9 mL headspace volume of the opical cell, nCO2 is amount of mol of CO2 measured in the headspace, p is the pressure in Pa (1 * 10 5 Pa), R the ideal gas constant (8.31 J K -1 mol -1 ) and T the temperature (293 K).Finally, cumulative CO2 release values of each cycle (Figure 5) were determined by subtracting the measured concentration at the beginning of light, from the concentration at the end of light exposure (Figure S13).

Figure S2 .
Figure S2.(A) Linear fit of the initial 10 minutes of MCH relaxation in the dark and (B) reversibility rate constant as a function of xDMSO assuming first order (linear) kinetics.

Figure S3 .
Figure S3.Custom-made optical cell (left), designed for photo-switching experiments.Photo-switching setup with optical cell, pH-meter, LED light and magnetic stirrer (right).

Figure S4 .
Figure S4.pH titration of PAH in (A) H2O and (B) xDMSO = 0.15, spectra increment with pH units of 0.5.(C) Absorption ratio between the absorption maximum (Amax) of MCH at 423nm, and MC-and SP peaks at 560 and 300 nm, respectively, of 0.08mM PAH in 20mM NaCl, with pH adjusted by mixtures of 10mM NaOH and 10mM HCl in water (black) and 15% DMSO (blue).
Figure S6.pH-jump of 0.08 mM MCH in different mole fractions of DMSO at the first light cycle, last light cycle (LC), and sample that remained in the dark at the same time of the last light cycle (D).Each cycle consists of 1 minute light and 58 minutes of darkness.

Figure S8 .
Figure S8.Solubility results of different methods: incremental (with pre-or post-mixed series) and UV-Vis (post-mixed).Average (all) are the same values as in Figure 4A.

Figure
Figure S10.(A) pH-jump of MCH in water at different concentrations and (B) corresponding released proton concentration as a function of photoacid concentration.

Figure
Figure S11.(A) pH-jump of MCH under continuous illumination in xDMSO = 0.15 at different concentrations, (B) corresponding pH-jump and (C) released proton concentration as a function of photoacid concentration.

Figure S13 .
Figure S13.Amount of CO2 detected in the headspace during 3 consecutive photochemical CO2 release cycles of light (yellow highlight) from concentrated MCH and molar equivalent KHCO3 in water (A) and in xDMSO = 0.15 (B).An N2 purge after each light cycle is indicated by dotted line.
q / e − , σ / Å, ε / kJ mol -1 , α / Å 3 .a Well-depth values of Lennard-Jones potential are provided before scaling.bAtomic polarizability of hydrogen atoms was merged onto the polarizability of the atoms to which they are bonded.

Figure S15 .
Figure S15.Radial distribution functions of oxygen atoms of water (left) and DMSO (right) around carbon atom of the double bond of merocyanine.

Figure S16 .
Figure S16.Radial distribution functions of oxygen atoms of water (left) and DMSO (right) around hydrogen atom of hydroxyl group of merocyanine.

Figure S17 .
Figure S17.Probability contours revealing hydrogen bonds of SP with water (left) and DMSO (right) in a system with xDMSO = 0.15.The x-axes represent the distances between the hydrogen atoms of a solvent and the acceptor oxygen atom of the solute (A).The y-axis represents the angles formed by the D-H...A hydrogen bonds, where D is a donor atom attached to the hydrogen.

Figure S18 .
Figure S18.Probability contours revealing hydrogen bonds of MCH with water (left) and DMSO (right) in a system with xDMSO = 0.15.The x-axes represent the distances between the hydrogen atom of the solute and the acceptor oxygen atoms of a solvent (A).The y-axis represents the angles formed by the D-H...A hydrogen bonds, where D is a donor atom attached to the hydrogen.

Table S2 .
Fitting parameters from calibration curves and resulting solubility values.

Table S3 .
MCH peak positions changing with DMSO mole fraction.XDMSO

Table S4 .
Force field parameters of merocyanine and spiropyran.The corresponding labels are given in FigureS14.

Table S5 .
Scaling coefficients for the modification of non-bonded attractive interactions